Worldwide, many users reported issues using Facebook, Instagram, Messenger, Oculus VR and WhatsApp. For some, the websites were down completely, while others struggled with limited functionality. User reports of Facebook being unreachable continued at Down Detector Thursday morning, while Instagram tweeted overnight that service had been restored.

Facebook had to go to Twitter (TWTR) to report that it was investigating the outage, which it said was not the result of a distributed denial of service, or DDoS, attack.

We're focused on working to resolve the issue as soon as possible, but can confirm that the issue is not related to a DDoS attack.

— Meta (@Meta) March 13, 2019

A Facebook executive told Wired that outside hackers were not to blame, and security experts concurred that there was no evidence of a malicious attack. More likely were any number of boring culprits, from routine maintenance gone wrong to a Domain Name System issue.

CNET reported the outage was the longest in Facebook’s 15-year history. The previous longest occurred in 2008, when Facebook had just 125 million users. Facebook and Instagram now have more than 2.3 billion and 1 billion users, respectively.

Secure messaging service Telegram got 3 million new users during the Facebook outage, TechCrunch reports. It’s reminiscent of the Friendster outages that sent users to MySpace.

The Facebook outage also affected the Ads Manager tool, which is infuriating to the 6 million advertisers who spend more than $90 million per day to reach potential customers, AdAge reports.

On Wednesday, Facebook was also facing another pressing issue: The New York Times reported federal prosecutors are conducting a criminal investigation into data deals Facebook struck with smartphone companies. A grand jury is subpoenaing records regarding the broad access to the personal information of hundreds of millions of Facebook users.